The development of techniques for effective and size selective chemical synthesis of colloidal dispersions of nanocrystals has led to significant advances in the 1990s. These nano-objects (fluorescent semiconductor nanocrystals, metallic nanocrystals, nanotubes, etc.) are nanosized crystals of pure semiconductor material (Si) or composed of type II-VI (CdSe), III-V (GaAs) or others, which, under ultraviolet light, re-emit a fluorescent light. The “color” (wavelength) depends on the size of the nanocrystal. Such emission is a result of the phenomenon called “quantum confinement”, which may be observed when the size of the nano-object is very small. More specifically, a nanoparticle may act as structure having discrete physicochemical properties when its size is less than or equal to the Bohr radius of the exciton. Above this radius, the nanoparticles act as inorganic materials having a band structure as shown in FIG. 1. In short, optical properties of nanoparticles (absorption and emission wavelengths) are generally linked to their composition (CdSe, ZnO, etc.) and controlled by their size and shape (sphere, rod, etc.) as shown in FIG. 1. Due to properties of nanoparticles (high absorption in the visible, photoluminescence, etc.), applications are very numerous (biological labeling, materials for light-emitting diodes and solar cells, etc.), and are similar to those of π-conjugated systems.
For example, optical properties of CdSe or CdS nanoparticles or their corresponding core-shell systems CdSe/CdS have attracted much attention during the last decade by the fact that their absorption and emission can be modulated over a large part of the visible spectrum by varying the size and structure of the nanoparticles. As shapes and sizes of CdSe, CdS and CdSe/CdS nanoparticles may be varied over a large range covering dots, rods, tetrapods and multipods of different sizes, their optical properties (absorption and emission wavelength) may also be modified. Generally, applications of CdSe or CdS based nanoparticles concern the field of photovoltaics, light-emitting diodes (LEDs), biology and nano-medicine. However, the toxicity of CdSe or CdS based nanoparticles makes their large-scale applications, such as in bio-medical fields, very difficult.
In contrast to CdSe or CdS based nanoparticles, other inorganic nanoparticles, such as metal oxides (e.g. ZnO nanoparticles, etc.) are known for their non-toxicity, low cost synthesis at large scale and the possibility to synthesize dots, rods, tetrapods and multipods of different sizes, similar to III-V nanoparticles. However, most inorganic nanoparticles, such as ZnO, have a very limited ability to vary their absorption and emission spectra in the visible region due to their wide bandgap. For example, although it is possible to obtain ZnO nanocrystals with emission in the visible region via creation of oxygen defects, this emission, however, needs excitation in the UV range and merely generates a weak fluorescence signal. Furthermore, emission spectrum of most inorganic nanoparticles cannot be modulated over a large range. For example, the emission of ZnO nanocrystals depends greatly on the environment, is not stable and can be extinguished completely.
Compared to nanoparticles, π-conjugated (e.g. semiconducting) organic systems are functional materials of interest for applications in less expensive and flexible electronic devices such as light-emitting diodes (OLEDs), field effect transistors (OFETs) and photovoltaic solar cells. This interest is mainly due to the possibility of modifying physical properties and supramolecular organization of π-conjugated organic systems via molecular structural variations. Predictive structure-property relationships may be established to suit a desired function via chemical engineering at the molecular level. For example, the development of advanced electroluminescent organic materials is possible following seminal reports of efficient organic light-emitting diodes (OLEDs) based on small molecules and conjugated polymers. However, varying the chemical composition of conjugated systems is a major concern to control their properties. Indeed, the optical and electronic properties of bulk material generally depend on the chemical structure of the conjugated monomeric/oligomeric/polymeric carbon backbone (HOMO-LUMO gap, electronic density, etc.) and on the interaction between the individual molecules (supramolecular arrangement, morphology). For example, luminescence processes for organic luminophores are generally concentration dependent. For most of the cases, the luminescence is weakened or totally quenched in concentrated solutions.
Since the last decade, a new field of so called hybrid nanomaterials has emerged that aims to combine the advantages of organic materials with inorganic nanocrystals allowing to generate new functionality via synergetic effects. Hybrid nanomaterials using ZnO nanoparticles as template were used in the past to assemble 1D and 2D nanoparticles with optical, electronic and photovoltaic properties. In this case, π-conjugated ligands were grafted onto the surface of ZnO nanoparticles of either spherical and rod-like shape leading to hybrid nanomaterials with opto-electronic properties governed by both the organic and inorganic component of the nanomaterial. For example, light absorption of ZnO nanoparticles could be increased in the visible via grafting organic dyes onto their surface. However fluorescence or phosphorescence emission of the dye that would allow the modulation of the emission properties of ZnO, are usually either quenched by exciton dissociation at the ZnO/dye interface due to the formation of a hybrid heterojunction or quenched due to the formation of dye aggregates at the surface of the inorganic component.
Accordingly, there exists a continuing need to provide luminescent hybrid nanomaterials; easy processes to manufacture the same; and thin films, luminescent solar concentrators, light-emitting hybrid diodes and light-emitting hybrid field-effect transistors comprising the same.